3.4.1 - The Cell Interior - The Cytoskeleton

The cytoskeleton is composed of what?

*Microfilaments, intermediate filaments and microtubules*.

Microfilaments form what?

A fibrous mat called the *terminal web* (membrane skeleton) on the cytoplasmic side of the plasma membrane.

Those three groups - the cytoskeleton, organelles, and inclusions - are embedded in what?

In the clear, gelatinous *cytosol*.

Keratin is most notably responsible for what?

The strength of your hair and fingernails.

Microfilaments are about ___ nm thick and are made of what?

6; the protein *actin*.

What is a microtubule?

A cylinder made of 13 parallel strands called protofilaments.

Each protofilament (component of microtubules) is comprised of what?

A long chain of globular proteins called *tubulin*.

What is the cytoskeleton and what is its function?

A network of protein filaments and cylinders that: • structurally *supports* the cell, • determines its *shape*, • *organizes* its contents, • *directs the movement* of materials within the cell, and • contributes to *movements* of the cell as a whole.

Through its role in cell motility, actin plays a crucial role in what processes?

Embryonic development, muscle contraction, immune function, wound healing, cancer metastasis, and other processes that involved cell migration.

Microtubules act somewhat like railroad tracks. Explain.

Motor proteins "walk" along these tracks carrying organelles and macromolecules to specific destinations in the cell.

Are microtubules permanent structures? Explain.

No; they come and go moment by moment as tubulin molecules assemble into a tubule and then suddenly break apart again to be used somewhere else in the cell. (However, the double and triple sets of microtubules in cilia, flagella, basal bodies, and centrioles are more stable)

What do microtubules do?

Radiate from an area of the cell called the *centrosome* and: • *hold* organelles in place, • form bundles that maintain *cell shape* and *rigidity*, and • act somewhat like *railroad tracks* for motor proteins.

Additionally, microtubules form what?

The *mitotic spindle* that guides chromosome movement during cell division.

Describe the connections of the cytoskeleton.

The cytoskeleton itself is connected to *transmembrane proteins* of the plasma membrane which are in turn connected to *protein fibers external* to the cell [creating a strong structural continuity from extracellular material to the cytoplasm].

The phospholipids of the plasma membrane spread out over the terminal web like butter on a slice of bread. Explain.

The web (made by the microfilaments), like the bread, provides *physical support*, whereas the lipids, like butter, provide a permeability barrier.

Microtubules also form the __________ of cilia and flagella which is responsible for what?

axonemes; their beating movements.

Cytoskeletal elements may even connect to __________ in the nucleus, enabling physical tension on a cell to do what?

chromosomes; To move nuclear contents and mechanically stimulate genetic function.

The cell's internal structures are classified into 3 groups:

cytoskeleton, organelles, and inclusions.

In epidermal cells, intermediate filaments are made of the tough protein __________ and occupy most of the cytoplasm.

keratin

A microtubule is (smaller/larger) than microfilaments and intermediate filaments ranging in size of what?

larger; 25 nm in diameter

Intermediate filaments are (smaller/larger) than microfilaments ranging in size of what?

larger; 8-10 nm in diameter (compared to 6 nm of microfilaments).

As described earlier, actin microfilaments also form the supportive cores of the __________ and play a role in what?

microvilli; cell movement.

What is the role of intermediate filaments in the cell?

• Shape • Resist stress • Participate in junctions that *attach cells to their neighbors*